As described in the IEEE 802.16m System Requirements, a femtocell is a low power base station (BS), typically installed by a subscriber in his/her home or small office/home office (SOHO) to provide access to closed or open group of users as configured by the subscriber and/or the access provider. Femtocell BSs typically operate in licensed spectrum and may use the same or different frequency as macro cells. Femtocells often use a broadband connection such as cable or DSL for backhaul. A mobile station (MS) that communicates in a femtocell is typically stationary or moving at low (i.e. pedestrian) speed.
Femtocells are different from macro cells in a number of ways. For example, the IEEE 802.16m System Requirements document lists the following specific requirements for femtocells that distinguish them from macro cells:
“The air interface shall support features needed to limit MS's scanning, access and handover to femtocell BS's with restricted access if they are designated as part of closed subscriber group (CSG).”
“The air interface shall support preferred access and handover of MS's to their designated femto-BS's.”
“The air interface should support optimized and seamless session continuity and handover between 16m Femtocell BS's and WiFi access systems.”
“The air interface shall allow dense deployment of large number of femtocells by an operator.”
The dense deployment of femtocells permitted by the IEEE 802.16m requirements may introduce substantial interference between a femtocell and a macro cell, and between multiple femtocells, especially in Closed Subscriber Groups (CSG), where a CSG femtocell only allows access to its own subscribers.